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Summary: Ctors | Methods | Protected Methods | Inherited Methods

# Hashtable

`public class Hashtable`
`` `extends Dictionary<K, V>` `implements Map<K, V>, Cloneable, Serializable`

java.lang.Object
↳ | java.util.Dictionary<K, V>
| ↳ | java.util.Hashtable<K, V>

Known direct subclasses Properties
Properties | The `Properties` class represents a persistent set of properties.

Known indirect subclasses AuthProvider, Provider
AuthProvider | Legacy security code; do not use.
Provider | This class represents a "provider" for the Java Security API, where a provider implements some or all parts of Java Security.

This class implements a hash table, which maps keys to values. Any non-`null` object can be used as a key or as a value.

To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the `hashCode` method and the `equals` method.

An instance of `Hashtable` has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. Note that the hash table is open: in the case of a "hash collision", a single bucket stores multiple entries, which must be searched sequentially. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. The initial capacity and load factor parameters are merely hints to the implementation. The exact details as to when and whether the rehash method is invoked are implementation-dependent.

Generally, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the time cost to look up an entry (which is reflected in most `Hashtable` operations, including `get` and `put`).

The initial capacity controls a tradeoff between wasted space and the need for `rehash` operations, which are time-consuming. No `rehash` operations will ever occur if the initial capacity is greater than the maximum number of entries the `Hashtable` will contain divided by its load factor. However, setting the initial capacity too high can waste space.

If many entries are to be made into a `Hashtable`, creating it with a sufficiently large capacity may allow the entries to be inserted more efficiently than letting it perform automatic rehashing as needed to grow the table.

This example creates a hashtable of numbers. It uses the names of the numbers as keys:

       Hashtable<String, Integer> numbers
         = new Hashtable<String, Integer>();
       numbers.put("one", 1);
       numbers.put("two", 2);
       numbers.put("three", 3);

To retrieve a number, use the following code:

       Integer n = numbers.get("two");
       if (n != null) {
         System.out.println("two = " + n);
       }

The iterators returned by the `iterator` method of the collections returned by all of this class's "collection view methods" are fail-fast: if the Hashtable is structurally modified at any time after the iterator is created, in any way except through the iterator's own `remove` method, the iterator will throw a `ConcurrentModificationException`. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by Hashtable's keys and elements methods are not fail-fast.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw `ConcurrentModificationException` on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

As of the Java 2 platform v1.2, this class was retrofitted to implement the `Map` interface, making it a member of the Java Collections Framework. Unlike the new collection implementations, `Hashtable` is synchronized. If a thread-safe implementation is not needed, it is recommended to use `HashMap` in place of `Hashtable`. If a thread-safe highly-concurrent implementation is desired, then it is recommended to use `ConcurrentHashMap` in place of `Hashtable`.

See also:

- `Object.equals(java.lang.Object)`
- `Object.hashCode()`
- `rehash()`
- `Collection`
- `Map`
- `HashMap`
- `TreeMap`

## Summary

### Public constructors
`Hashtable(int initialCapacity, float loadFactor)` Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.
`Hashtable(int initialCapacity)` Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75).
`Hashtable()` Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75).
`Hashtable(Map<? extends K, ? extends V> t)` Constructs a new hashtable with the same mappings as the given Map.

### Public methods
`void` | `clear()` Clears this hashtable so that it contains no keys.
`Object` | `clone()` Creates a shallow copy of this hashtable.
`V` | `compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)` Attempts to compute a mapping for the specified key and its current mapped value (or `null` if there is no current mapping).
`V` | `computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)` If the specified key is not already associated with a value (or is mapped to `null`), attempts to compute its value using the given mapping function and enters it into this map unless `null`.
`V` | `computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)` If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
`boolean` | `contains(Object value)` Tests if some key maps into the specified value in this hashtable.
`boolean` | `containsKey(Object key)` Tests if the specified object is a key in this hashtable.
`boolean` | `containsValue(Object value)` Returns true if this hashtable maps one or more keys to this value.
`Enumeration<V>` | `elements()` Returns an enumeration of the values in this hashtable.
`Set<Entry<K, V>>` | `entrySet()` Returns a `Set` view of the mappings contained in this map.
`boolean` | `equals(Object o)` Compares the specified Object with this Map for equality, as per the definition in the Map interface.
`void` | `forEach(BiConsumer<? super K, ? super V> action)` Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
`V` | `get(Object key)` Returns the value to which the specified key is mapped, or `null` if this map contains no mapping for the key.
`V` | `getOrDefault(Object key, V defaultValue)` Returns the value to which the specified key is mapped, or `defaultValue` if this map contains no mapping for the key.
`int` | `hashCode()` Returns the hash code value for this Map as per the definition in the Map interface.
`boolean` | `isEmpty()` Tests if this hashtable maps no keys to values.
`Set<K>` | `keySet()` Returns a `Set` view of the keys contained in this map.
`Enumeration<K>` | `keys()` Returns an enumeration of the keys in this hashtable.
`V` | `merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)` If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
`V` | `put(K key, V value)` Maps the specified `key` to the specified `value` in this hashtable.
`void` | `putAll(Map<? extends K, ? extends V> t)` Copies all of the mappings from the specified map to this hashtable.
`V` | `putIfAbsent(K key, V value)` If the specified key is not already associated with a value (or is mapped to `null`) associates it with the given value and returns `null`, else returns the current value.
`boolean` | `remove(Object key, Object value)` Removes the entry for the specified key only if it is currently mapped to the specified value.
`V` | `remove(Object key)` Removes the key (and its corresponding value) from this hashtable.
`boolean` | `replace(K key, V oldValue, V newValue)` Replaces the entry for the specified key only if currently mapped to the specified value.
`V` | `replace(K key, V value)` Replaces the entry for the specified key only if it is currently mapped to some value.
`void` | `replaceAll(BiFunction<? super K, ? super V, ? extends V> function)` Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.
`int` | `size()` Returns the number of keys in this hashtable.
`String` | `toString()` Returns a string representation of this `Hashtable` object in the form of a set of entries, enclosed in braces and separated by the ASCII characters "`,` " (comma and space).
`Collection<V>` | `values()` Returns a `Collection` view of the values contained in this map.

### Protected methods
`void` | `rehash()` Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently.

### Inherited methods
From class `java.util.Dictionary`
`abstract Enumeration<V>` | `elements()` Returns an enumeration of the values in this dictionary.
`abstract V` | `get(Object key)` Returns the value to which the key is mapped in this dictionary.
`abstract boolean` | `isEmpty()` Tests if this dictionary maps no keys to value.
`abstract Enumeration<K>` | `keys()` Returns an enumeration of the keys in this dictionary.
`abstract V` | `put(K key, V value)` Maps the specified `key` to the specified `value` in this dictionary.
`abstract V` | `remove(Object key)` Removes the `key` (and its corresponding `value`) from this dictionary.
`abstract int` | `size()` Returns the number of entries (distinct keys) in this dictionary.
From class `java.lang.Object`
`Object` | `clone()` Creates and returns a copy of this object.
`boolean` | `equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void` | `finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>` | `getClass()` Returns the runtime class of this `Object`.
`int` | `hashCode()` Returns a hash code value for the object.
`final void` | `notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void` | `notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String` | `toString()` Returns a string representation of the object.
`final void` | `wait(long timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.
From interface `java.util.Map`
`abstract void` | `clear()` Removes all of the mappings from this map (optional operation).
`default V` | `compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)` Attempts to compute a mapping for the specified key and its current mapped value (or `null` if there is no current mapping).
`default V` | `computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)` If the specified key is not already associated with a value (or is mapped to `null`), attempts to compute its value using the given mapping function and enters it into this map unless `null`.
`default V` | `computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)` If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
`abstract boolean` | `containsKey(Object key)` Returns `true` if this map contains a mapping for the specified key.
`abstract boolean` | `containsValue(Object value)` Returns `true` if this map maps one or more keys to the specified value.
`static <K, V> Entry<K, V>` | `entry(K k, V v)` Returns an immutable `Entry` containing the given key and value.
`abstract Set<Entry<K, V>>` | `entrySet()` Returns a `Set` view of the mappings contained in this map.
`abstract boolean` | `equals(Object o)` Compares the specified object with this map for equality.
`default void` | `forEach(BiConsumer<? super K, ? super V> action)` Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
`abstract V` | `get(Object key)` Returns the value to which the specified key is mapped, or `null` if this map contains no mapping for the key.
`default V` | `getOrDefault(Object key, V defaultValue)` Returns the value to which the specified key is mapped, or `defaultValue` if this map contains no mapping for the key.
`abstract int` | `hashCode()` Returns the hash code value for this map.
`abstract boolean` | `isEmpty()` Returns `true` if this map contains no key-value mappings.
`abstract Set<K>` | `keySet()` Returns a `Set` view of the keys contained in this map.
`default V` | `merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)` If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
`static <K, V> Map<K, V>` | `of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5)` Returns an immutable map containing five mappings.
`static <K, V> Map<K, V>` | `of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4)` Returns an immutable map containing four mappings.
`static <K, V> Map<K, V>` | `of(K k1, V v1)` Returns an immutable map containing a single mapping.
`static <K, V> Map<K, V>` | `of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8)` Returns an immutable map containing eight mappings.
`static <K, V> Map<K, V>` | `of(K k1, V v1, K k2, V v2)` Returns an immutable map containing two mappings.
`static <K, V> Map<K, V>` | `of(K k1, V v1, K k2, V v2, K k3, V v3)` Returns an immutable map containing three mappings.
`static <K, V> Map<K, V>` | `of()` Returns an immutable map containing zero mappings.
`static <K, V> Map<K, V>` | `of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9)` Returns an immutable map containing nine mappings.
`static <K, V> Map<K, V>` | `of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6)` Returns an immutable map containing six mappings.
`static <K, V> Map<K, V>` | `of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7)` Returns an immutable map containing seven mappings.
`static <K, V> Map<K, V>` | `of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10)` Returns an immutable map containing ten mappings.
`static <K, V> Map<K, V>` | `ofEntries(Entry...<? extends K, ? extends V> entries)` Returns an immutable map containing keys and values extracted from the given entries.
`abstract V` | `put(K key, V value)` Associates the specified value with the specified key in this map (optional operation).
`abstract void` | `putAll(Map<? extends K, ? extends V> m)` Copies all of the mappings from the specified map to this map (optional operation).
`default V` | `putIfAbsent(K key, V value)` If the specified key is not already associated with a value (or is mapped to `null`) associates it with the given value and returns `null`, else returns the current value.
`default boolean` | `remove(Object key, Object value)` Removes the entry for the specified key only if it is currently mapped to the specified value.
`abstract V` | `remove(Object key)` Removes the mapping for a key from this map if it is present (optional operation).
`default boolean` | `replace(K key, V oldValue, V newValue)` Replaces the entry for the specified key only if currently mapped to the specified value.
`default V` | `replace(K key, V value)` Replaces the entry for the specified key only if it is currently mapped to some value.
`default void` | `replaceAll(BiFunction<? super K, ? super V, ? extends V> function)` Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.
`abstract int` | `size()` Returns the number of key-value mappings in this map.
`abstract Collection<V>` | `values()` Returns a `Collection` view of the values contained in this map.

## Public constructors

### Hashtable

    public Hashtable (int initialCapacity,
                    float loadFactor)

Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.

Parameters
`initialCapacity` | `int`: the initial capacity of the hashtable.
`loadFactor` | `float`: the load factor of the hashtable.

Throws
`IllegalArgumentException` | if the initial capacity is less than zero, or if the load factor is nonpositive.

### Hashtable

    public Hashtable (int initialCapacity)

Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75).

Parameters
`initialCapacity` | `int`: the initial capacity of the hashtable.

Throws
`IllegalArgumentException` | if the initial capacity is less than zero.

### Hashtable

    public Hashtable ()

Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75).

### Hashtable

    public Hashtable (Map<? extends K, ? extends V> t)

Constructs a new hashtable with the same mappings as the given Map. The hashtable is created with an initial capacity sufficient to hold the mappings in the given Map and a default load factor (0.75).

Parameters
`t` | `Map`: the map whose mappings are to be placed in this map.

Throws
`NullPointerException` | if the specified map is null.

## Public methods

### clear

    public void clear ()

Clears this hashtable so that it contains no keys.

### clone

    public Object clone ()

Creates a shallow copy of this hashtable. All the structure of the hashtable itself is copied, but the keys and values are not cloned. This is a relatively expensive operation.

Returns
`Object` | a clone of the hashtable

### compute

    public V compute (K key,
                    BiFunction<? super K, ? super V, ? extends V> remappingFunction)

Attempts to compute a mapping for the specified key and its current mapped value (or `null` if there is no current mapping). For example, to either create or append a `String` msg to a value mapping:

     map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))

(Method `merge()` is often simpler to use for such purposes.)

If the remapping function returns `null`, the mapping is removed (or remains absent if initially absent). If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The remapping function should not modify this map during computation.

Parameters
`key` | `K`: key with which the specified value is to be associated
`remappingFunction` | `BiFunction`: the remapping function to compute a value

Returns
`V` | the new value associated with the specified key, or null if none

### computeIfAbsent

    public V computeIfAbsent (K key,
                    Function<? super K, ? extends V> mappingFunction)

If the specified key is not already associated with a value (or is mapped to `null`), attempts to compute its value using the given mapping function and enters it into this map unless `null`.

If the mapping function returns `null`, no mapping is recorded. If the mapping function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded. The most common usage is to construct a new object serving as an initial mapped value or memoized result, as in:

     map.computeIfAbsent(key, k -> new Value(f(k)));

Or to implement a multi-value map, `Map<K,Collection<V>>`, supporting multiple values per key:

     map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);

The mapping function should not modify this map during computation.

Parameters
`key` | `K`: key with which the specified value is to be associated
`mappingFunction` | `Function`: the mapping function to compute a value

Returns
`V` | the current (existing or computed) value associated with the specified key, or null if the computed value is null

### computeIfPresent

    public V computeIfPresent (K key,
                    BiFunction<? super K, ? super V, ? extends V> remappingFunction)

If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.

If the remapping function returns `null`, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The remapping function should not modify this map during computation.

Parameters
`key` | `K`: key with which the specified value is to be associated
`remappingFunction` | `BiFunction`: the remapping function to compute a value

Returns
`V` | the new value associated with the specified key, or null if none

### contains

    public boolean contains (Object value)

Tests if some key maps into the specified value in this hashtable. This operation is more expensive than the `containsKey` method.

Note that this method is identical in functionality to `containsValue`, (which is part of the `Map` interface in the collections framework).

Parameters
`value` | `Object`: a value to search for

Returns
`boolean` | `true` if and only if some key maps to the `value` argument in this hashtable as determined by the `equals` method; `false` otherwise.

Throws
`NullPointerException` | if the value is `null`

### containsKey

    public boolean containsKey (Object key)

Tests if the specified object is a key in this hashtable.

Parameters
`key` | `Object`: possible key

Returns
`boolean` | `true` if and only if the specified object is a key in this hashtable, as determined by the `equals` method; `false` otherwise.

Throws
`NullPointerException` | if the key is `null`

See also:

- `contains(Object)`

### containsValue

    public boolean containsValue (Object value)

Returns true if this hashtable maps one or more keys to this value.

Note that this method is identical in functionality to `contains` (which predates the `Map` interface).

Parameters
`value` | `Object`: value whose presence in this hashtable is to be tested

Returns
`boolean` | `true` if this map maps one or more keys to the specified value

Throws
`NullPointerException` | if the value is `null`

### elements

    public Enumeration<V> elements ()

Returns an enumeration of the values in this hashtable. Use the Enumeration methods on the returned object to fetch the elements sequentially.

Returns
`Enumeration<V>` | an enumeration of the values in this hashtable.

See also:

- `Enumeration`
- `keys()`
- `values()`
- `Map`

### entrySet

    public Set<Entry<K, V>> entrySet ()

Returns a `Set` view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own `remove` operation, or through the `setValue` operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the `Iterator.remove`, `Set.remove`, `removeAll`, `retainAll` and `clear` operations. It does not support the `add` or `addAll` operations.

Returns
`Set<Entry<K, V>>` | a set view of the mappings contained in this map

### equals

    public boolean equals (Object o)

Compares the specified Object with this Map for equality, as per the definition in the Map interface.

Parameters
`o` | `Object`: object to be compared for equality with this hashtable

Returns
`boolean` | true if the specified Object is equal to this Map

See also:

- `Map.equals(Object)`

### forEach

    public void forEach (BiConsumer<? super K, ? super V> action)

Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. Unless otherwise specified by the implementing class, actions are performed in the order of entry set iteration (if an iteration order is specified.) Exceptions thrown by the action are relayed to the caller.

Parameters
`action` | `BiConsumer`: The action to be performed for each entry

### get

    public V get (Object key)

Returns the value to which the specified key is mapped, or `null` if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key `k` to a value `v` such that `(key.equals(k))`, then this method returns `v`; otherwise it returns `null`. (There can be at most one such mapping.)

Parameters
`key` | `Object`: the key whose associated value is to be returned

Returns
`V` | the value to which the specified key is mapped, or `null` if this map contains no mapping for the key

Throws
`NullPointerException` | if the specified key is null

See also:

- `put(Object, Object)`

### getOrDefault

    public V getOrDefault (Object key,
                    V defaultValue)

Returns the value to which the specified key is mapped, or `defaultValue` if this map contains no mapping for the key.

Parameters
`key` | `Object`: the key whose associated value is to be returned
`defaultValue` | `V`: the default mapping of the key

Returns
`V` | the value to which the specified key is mapped, or `defaultValue` if this map contains no mapping for the key

### hashCode

    public int hashCode ()

Returns the hash code value for this Map as per the definition in the Map interface.

Returns
`int` | a hash code value for this object.

See also:

- `Map.hashCode()`

### isEmpty

    public boolean isEmpty ()

Tests if this hashtable maps no keys to values.

Returns
`boolean` | `true` if this hashtable maps no keys to values; `false` otherwise.

### keySet

    public Set<K> keySet ()

Returns a `Set` view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own `remove` operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the `Iterator.remove`, `Set.remove`, `removeAll`, `retainAll`, and `clear` operations. It does not support the `add` or `addAll` operations.

Returns
`Set<K>` | a set view of the keys contained in this map

### keys

    public Enumeration<K> keys ()

Returns an enumeration of the keys in this hashtable.

Returns
`Enumeration<K>` | an enumeration of the keys in this hashtable.

See also:

- `Enumeration`
- `elements()`
- `keySet()`
- `Map`

### merge

    public V merge (K key,
                    V value,
                    BiFunction<? super V, ? super V, ? extends V> remappingFunction)

If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result is `null`. This method may be of use when combining multiple mapped values for a key. For example, to either create or append a `String msg` to a value mapping:

     map.merge(key, msg, String::concat)

If the remapping function returns `null`, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The remapping function should not modify this map during computation.

Parameters
`key` | `K`: key with which the resulting value is to be associated
`value` | `V`: the non-null value to be merged with the existing value associated with the key or, if no existing value or a null value is associated with the key, to be associated with the key
`remappingFunction` | `BiFunction`: the remapping function to recompute a value if present

Returns
`V` | the new value associated with the specified key, or null if no value is associated with the key

### put

    public V put (K key,
                    V value)

Maps the specified `key` to the specified `value` in this hashtable. Neither the key nor the value can be `null`.

The value can be retrieved by calling the `get` method with a key that is equal to the original key.

Parameters
`key` | `K`: the hashtable key
`value` | `V`: the value

Returns
`V` | the previous value of the specified key in this hashtable, or `null` if it did not have one

Throws
`NullPointerException` | if the key or value is `null`

See also:

- `Object.equals(Object)`
- `get(Object)`

### putAll

    public void putAll (Map<? extends K, ? extends V> t)

Copies all of the mappings from the specified map to this hashtable. These mappings will replace any mappings that this hashtable had for any of the keys currently in the specified map.

Parameters
`t` | `Map`: mappings to be stored in this map

Throws
`NullPointerException` | if the specified map is null

### putIfAbsent

    public V putIfAbsent (K key,
                    V value)

If the specified key is not already associated with a value (or is mapped to `null`) associates it with the given value and returns `null`, else returns the current value.

Parameters
`key` | `K`: key with which the specified value is to be associated
`value` | `V`: value to be associated with the specified key

Returns
`V` | the previous value associated with the specified key, or `null` if there was no mapping for the key. (A `null` return can also indicate that the map previously associated `null` with the key, if the implementation supports null values.)

### remove

    public boolean remove (Object key,
                    Object value)

Removes the entry for the specified key only if it is currently mapped to the specified value.

Parameters
`key` | `Object`: key with which the specified value is associated
`value` | `Object`: value expected to be associated with the specified key

Returns
`boolean` | `true` if the value was removed

### remove

    public V remove (Object key)

Removes the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.

Parameters
`key` | `Object`: the key that needs to be removed

Returns
`V` | the value to which the key had been mapped in this hashtable, or `null` if the key did not have a mapping

Throws
`NullPointerException` | if the key is `null`

### replace

    public boolean replace (K key,
                    V oldValue,
                    V newValue)

Replaces the entry for the specified key only if currently mapped to the specified value.

Parameters
`key` | `K`: key with which the specified value is associated
`oldValue` | `V`: value expected to be associated with the specified key
`newValue` | `V`: value to be associated with the specified key

Returns
`boolean` | `true` if the value was replaced

### replace

    public V replace (K key,
                    V value)

Replaces the entry for the specified key only if it is currently mapped to some value.

Parameters
`key` | `K`: key with which the specified value is associated
`value` | `V`: value to be associated with the specified key

Returns
`V` | the previous value associated with the specified key, or `null` if there was no mapping for the key. (A `null` return can also indicate that the map previously associated `null` with the key, if the implementation supports null values.)

### replaceAll

    public void replaceAll (BiFunction<? super K, ? super V, ? extends V> function)

Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception. Exceptions thrown by the function are relayed to the caller.

Parameters
`function` | `BiFunction`: the function to apply to each entry

### size

    public int size ()

Returns the number of keys in this hashtable.

Returns
`int` | the number of keys in this hashtable.

### toString

    public String toString ()

Returns a string representation of this `Hashtable` object in the form of a set of entries, enclosed in braces and separated by the ASCII characters "`,` " (comma and space). Each entry is rendered as the key, an equals sign `=`, and the associated element, where the `toString` method is used to convert the key and element to strings.

Returns
`String` | a string representation of this hashtable

### values

    public Collection<V> values ()

Returns a `Collection` view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress (except through the iterator's own `remove` operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the `Iterator.remove`, `Collection.remove`, `removeAll`, `retainAll` and `clear` operations. It does not support the `add` or `addAll` operations.

Returns
`Collection<V>` | a collection view of the values contained in this map

## Protected methods

### rehash

    protected void rehash ()

Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently. This method is called automatically when the number of keys in the hashtable exceeds this hashtable's capacity and load factor.

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